Process of flexographic printing utilizing an electrical field

ABSTRACT

A process for flexographic printing in which ink transfer between the plate cylinder and the web which passes over the impression cylinder is controlled by an applied electrical field between the impression cylinder and the plate cylinder in a direction such that a polarity sensitive flexographic printing ink is moved away from the web towards the plate cylinder whereby only contact areas between the plate cylinder image and the web will receive printing ink due to the mechanical contact.

0 United States Patent 1151 3,661,08 1 Wright 1 1 May 9, 1972 [54] PROCESS OF FLEXOGRAPHIC 2,558,901 7/1951 Hooper ..101/219 PRINTING UTILIZING AN 3,460,475 8/1969 Brown et al 101/416 X ELECTRICAL FIELD guelgnerw. ...l0l(/)2lg X ue ner ..l l/ 19 1 1 luvs-mom L Wright, Tranmere. Australia 2,558,900 7/1951 Hooper ..101/219 [73] Assignee: Hurletron Controls Division, Danville, lll.

Primary Examiner-Robert E. Pulfrey [22] Fled: 221 1969 Assistant Examiner-Cliff0rd D. Crowder [21 1 Appl No; 870,525 Attorney-Kinzer, Dorn and Zickert 57 ABSTRACT [30] Foreign Application Priority Data 1 A process for flexographic printing in which ink transfer NOV. 1, 1968 Australla ..45744 between the plate cylinder and h web which passes Over he impression cylinder is controlled by an applied electrical field [2%] }J.S.((:Z|l. ..101/426, 101/219, 101/416R between the impression cylinder and the plate cylinder in a f i direction such that a polarity sensitive flexographic printing I 1 le 0 arc l ink is moved away from the web towards the plate cylinder whereby only contact areas between the plate cylinder image [56] References Cned and the web will receive printing ink due to the mechanical UNITED STATES PATENTS Contact Pr/nled hinge flee/rode.

/mpresslon C /inder Source Worth ..lOl/2l9 4 Claims, 1 Drawing Figure PROCESS OF FLEXOGRAPHIC PRINTING UTILIZING AN ELECTRICAL FIELD This invention relates to printing and in particular relates to a method whereby the sharpness and clarity of images printed by flexographic methods may be improved considerably over that obtainable using presently known methods, by the application of an electric field at the printing nip in a manner not hitherto known, whereby such applied field retards the deposition of printing ink from the plate to the web or sheet being printed, and in particular inhibits ink transfer from the nonprinting areas of the printing plate.

Flexographic printing is a known method of reproduction in which the master or printing plate is in the form of a relatively flexible impression of the original to be reproduced. Such flexible relief plates contain as raised areas the image portions which are to retain ink which is subsequently transferred to a desired surface, such surface commonly being that of a paper web or a continuous plastic or other synthetic film. Flexographic printing is also used to reproduce information on relatively rough surfaces such as corrugated cardboard and the like.

Flexographic inks are normally described as thin inks, or relatively low viscosity inks, by which it is understood that the ink is sufficiently fluid in character to flow in a bath and assume a comparatively level surface. This allows the use of an ink distributing system which consists usually of two rollers, one of which, known as the fountain roller, is partially immersed in the bath of ink, and which rotates in contact with another roller, known as the form roller, which form roller is arranged to touch the surface of the flexible printing plate and transfer ink thereto. A web of material to be printed is brought into contact with the ink printing plate, usually by passage between the printing plate and a roller known as the impression cylinder. For convenience the printing plate is usually wrapped around a cylinder known as. the plate cylinder and the gap between such plate cylinder and the impression cylinder is adjusted to allow passage of the paper or other web through the gap in such manner that the web may touch each cylinder simultaneously. It is generally found necessary to so arrange the gap between the plate and impression cylinders to allow passage of the web without compressing the printing plate, in order to minimize distortion of the flexible plate and of the subsequent printed image. Thus, extremely low printing pressures are required and these necessitate very accurate makeready in order that all parts of the printing surface may contact the form roller and the web being printed. it is therefore a fairly common practice to increase the printing pressure in order to achieve the desired contact over the entire printing surface. However, because of the relatively liquid nature of the inks used it is common for printing plates to hold ink in the non-printing lower areas as well as on the raised printing surface and consequently areas of localized high printing pressures will often allow ink transfer from said lower non-image portions of the printing surface, which, as will be realized, gives rise to image distortion and/or substantial decrease in resolution. lt has thus become customary to limit the use of flexography to those occasions where the reproduction of fine detail is not required, and thus flexography is not used when type finer than 6 point is to be reproduced, and likewise it is not used for the reproduction of screened images finer than 100 lines/inch.

Electrostatic methods of assisting ink transfer are known, such as for example, those of Huebner as revealed, for example, in US. Pat. Nos. 2,483,462, 2,451,288, and 2,468,400 and those of Hooper, US. Pat. Nos. 2,558,900 and 2,520,504. More recently electrostatic assistance has been adapted to gravure printing in which the application of an electrostatic field across the printing nip has been claimed to draw the ink from the gravure cells and assist the transfer of such ink to paper.

The present invention also uses an electrostatic force at the printing nip but the action of such force is contrary to the principles of Huebner, Hooper and the more recent gravure processes, in that the electrostatic field in accordance with this invention is employed to suppress or hold back or depress the ink in the non-printing lower areas of the plate and thereby to prevent its transfer to the paper or other web in order to substantially eliminate image distortion and to increase resolution. By this means it has been found possible, for instance, to obtain excellent reproductions of line screen images using the normal fluid flexographic inks.

Broadly, this invention may be put into practice by the provision of electrical charging means which may be applied to the impression cylinder whilst grounding the plate cylinder whereby an electrostatic field can be established across the nip. Such charging means may consist simply of a corona charging wire directed to place an electrostatic charge on an insulating layer forming the outer surface of the impression cylinder or alternatively, such outer surface of the impression cylinder may be a conductor or a semiconductor insulated from ground having the potential applied thereto directly or by contact with a roller or other device whereby an electrostatic field is established across the nip.

lt will be realized that flexographic inks may be of varying chemical compositions and physical properties. Such inks normally contain a pigment or dye or a mixture of pigments and dyes dispersed in a liquid phase, with or without a resinous or other binder. The liquid phase may be an alcohol, such as methanol, either alone or mixed with other solvents such as propyl acetate and the like, or alternatively, the liquid phase may contain substantial quantities of water, or it may be a non-polar hydrocarbon solvent, and thus the electrical properties of such inks may be expected to show wide variation, in that some will be attracted to a positive polarity and others will be attracted to a negative polarity. Thus, it is desirable that the equipment used in accordance with this invention to generate the previously mentioned electrostatic field or force should be capable of delivering high voltage output which is either positive or negative with respect to ground, in order that the potential applied to the impression cylinder may have the polarity opposite to that which is capable of attracting the particular ink being used.

This invention also relies for its effectiveness on the electrical conductivity of the flexible relief plate, and we have found that in order to obtain the required repelling field effect using relatively easily provided high voltages the flexible relief plate should preferably have a through resistivity not in excess of 7.5 X l0 ohms, when such resistivity is measured using electrodes of 1 sq. cm. area placed one on each side of the plate and contacting the plate.

Thus the present invention provides a process for flexographic printing wherein a flexible relief plate with an electrical resistivity as previously defined is positioned on a grounded plate cylinder, wherein an impression cylinder is positioned to ensure contact between the thus positioned plate and the web being printed, characterized by the contact surface of such impression cylinder being insulated from ground, wherein a polarity sensitive flexographic printing ink is applied to the flexible relief plate and transferred to the web under the influence of a direct current electrical field, further characterized by such field being applied between the impression surface of the impression cylinder and the grounded flexible relief plate in such a direction that it retards the transfer of the polarity sensitive ink from the flexible relief plate to the web being printed, whereby the tendency for ink to transfer from the non-printing areas of the plate to the web is reduced by the repressive influence of the applied directional field opposing migration of ink in the low areas in the plate surrounding the relief characters.

lt will be realized that this present invention depends for its effectiveness in addition on the polarity sensitivity of the printing ink being used. At the present time commercially available inks are not classified with respect to the polarity selectivity, and the following method has been found advantageous for determining the polarity characteristics of any particular ink which is required to be used in accordance with the principles of the herein described improved flexographic printing process. A direct current voltage supply with variable output is provided with output leads connected to each of the positive and negative output terminals of such voltage supply. The voltage supply unit should preferably be so constructed that it can provide a variable output of 100-5000 volts at a current of up to about 5 milliamps. Either the positive or the negative output terminal may be grounded if desired. The output terminals are connected by the output leads to two metal plate electrodes, insulated from each other, which electrodes are substantially flat plates, parallel to each other, and spaced V; to 2 inches apart. The electrodes are immersed in a sample of the ink to be tested, which sample is contained preferably in an insulating container, such as a glass beaker or the like, and the power supply is switched on for a short time, such as l-60 seconds, the voltage being adjusted in accordance with the ink type and electrode spacing, for example 5,000 volts would be applied to electrodes 1 inch apart immersed in an insulating ink such as an ink containing a hydrocarbon solvent as the major liquid component, whereas the voltage would be reduced to 100-500 volts with an aqueous ink. After the power has been switched off the electrodes are removed from the system and examined. A polarity sensitive ink will show preferential deposition on one or other of the electrode, such ink is used in accordance with the present invention with a negative polarity applied to the impression surface of the impression cylinder, with the plate cylinder grounded.

In order that the invention may be more fully understood, reference will now be made to the accompanying drawing designated FIG. 1, which is a schematic representation of a printing unit on a flexographic printing press in accordance with the principles of this present invention.

A paper or other web 1 to be printed, is passed around a cylinder 2, known as an impression cylinder, the position of which cylinder is adjusted in relation to the plate cylinder 3 to allow the raised areas 5 of a flexible relief printing plate 4 to contact the surface of the web 1 while the depressed areas 6 of such flexible relief plate 4 do not contact the surface of the web 1.

An inking system is provided whereby ink is transferred to the surface of the flexible relief plate 4, such inking system consisting of a form roller 9 which may be etched or engraved on its outer surface whereby the quantity of ink transported by such surface is controlled, which form roller 9 is so positioned that it contacts the surface of the flexible relief plate 4 and in addition contacts the surface of a fountain roller 8, which fountain roller is at least partially immersed in a bath of liquid ink 7.

When the printing station is energized to cause rotation of the various rollers and cylinders, a clockwise rotation of fountain roller 8 lifts a layer of ink which is transferred to the form roller 9 which is rotating in an anti-clockwise direction. The plate cylinder 3 rotates in a clockwise direction, and the film of ink on the surface of the form roller 9 is transferred to the surface of the flexible relief plate 4 where it forms ink film 10.

As will be seen from the illustration, the ink film 10 is formed on the raised printing areas 5 and the depressed nonprinting areas 6 of the flexible relief plate 4.

As the form roller 3 rotates in a clockwise direction the inked portions of the plate are brought progressively into the printing zone, at which zone the paper web 1 contacts the raised portion 5 of the flexible relief plate 4, and when prior art methods are used the ink transfers from the depressed portions of the plate to the paper in addition to transferring from the raised printing areas. The additions taught by this present invention involve the grounding of the plate cylinder as shown in the illustration and the provision of a high voltage source HT connected to electrode 11 which electrode contacts the surface of the impression cylinder 2, which surface is isolated from ground.

When the high voltage source HT is energized, a field 13 is set up across the printing nip, which field is in a direction which retards ink transfer from the flexible relief plate to the paper, whereby the ink in the non-printing depressed areas 6 of the flexible relief plate 4 is retained by such surface and prevented from transferring to the web 1. Thus the printed deposit 12 on the web corresponds in size and shape with the raised printing areas 5 on the flexible relief plate 4.

The following examples are given to still further aid the understanding of the invention. In these examples the polarity sensitive flexographic printing inks with polarity sensitive characteristics determined in accordance with the method previously described were used in conjunction with flexible relief printing plates with resistivities determined with the electrode arrangement previously defined, under the influence of an electric field directed to retard transfer of the ink from the flexible relief plate to the web being printed. It will be realized that other flexographic inks and other flexible relief plates used for printing other web materials will in many instances require the application of different direct current voltages, but provided the electric field is applied in the repelling sense as previously defined such application will be within the scope of the present invention, which is therefore not limited in scope to the examples now following.

EXAMPLE 1 In a flexographic printing press, the impression cylinder was provided with an insulating sleeve, which sleeve was covered with a layer of relatively resilient semi-conducting synthetic rubber, the outer surface of which formed the impression surface. A conducting roller was positioned in contact with the semi-conducting surface of such impression cylinder, which conducting roller was connected to a terminal on a direct current variable high voltage power supply. The power supply was so arranged that either the positive or the negative output terminal could be connected to the conducting roller as desired, with the other terminal being grounded. The resistivity of the insulating sleeve, measured as previously described, was of the order of 10 ohms. The resistivity of the semi-conducting resilient synthetic rubber outer layer measured as described was 10 ohms.

A flexible relief plate was positioned on the grounded plate cylinder of the flexographic press. This plate was of acrylonitrile synthetic rubber, with a resistivity measured as described of 1.7 X 10* ohms.

A water reducible binder for flexographic printing ink was prepared by dissolving 250 grams of casein in 10000 grams of distilled water.

A water reducible binder for flexographic printing ink was prepared by dissolving 250 grams of casein in 1,000 grams of distilled water.

A flexographic printing ink was then prepared by dispersing Hostaperm Blue AR Colanyl in Casein binder grams 250 grams tive terminal was connected to the plate cylinder, and in addition the positive terminal was grounded. With the power supply switched off a ISO-line screen subject printed on paper with poor definition and in particular showed heavy scumming, which was particularly noticeable in highlight areas. Typescript finer than 6 point was generally of poor definition.

A further printing run was carried out using the repelling field, by the application of 500 volts negative to the conducting roller in contact with the impression surface of the impression cylinder. The repelling field gave a considerable reduc- EXAMPLE 2 The acrylonitrile rubber printing plate of Example 1 was replaced with a magnetic iron oxide filled natural rubber plate, with a resistivity measured as previously described of 7.5 X ohms. In this instance improved definition and reduction in scumming were achieved when the negative voltage applied to the conducting roller contacting the impression cylinder was raised to 6,000 volts. The relatively higher voltage was found to be necessary because of the higher resistivity of the printing plate.

EXAMPLE 3 The acrylonitrile rubber printing plate of Example 1 was replaced with a nitrile rubber plate, resistivity 3 x 10 ohms when measured as previously described. In this instance the negative voltage applied to the conductive roller in contact with the impression surface of the impression cylinder was 1,500 volts. The same improvement in definition and reduction in scumming was obtained.

EXAMPLE 4 The acrylonitrile rubber printing plate of Example 1 was replaced with a carbon filled natural rubber printing plate with a resistivity of 2 X l0 ohms when measured as previously described. Beneficial results as in Example 1 were obtained with the application of 300 volts negative to the conducting roller in contact with the impression surface of the impression cylinder.

EXAMPLES 5 to 8 The Hostaperm Blue AR Colanyl pigment of Examples 1 to 4 was replaced with an equal weight of Hosta perm black PR Colanyl, Color Index Pigment Black 6, by Hoechst. This pigment is an aqueous dispersion of carbon black containing 50 percent solids.

EXAMPLES 9 to 12 The Hostaperm Blue AR Colanyl pigment of Examples l to 4 was replaced with an equal weight of l-lansa Scarlet RNZ Colanyl, Color Index Pigment Red 3, by Hoechst. This is an aqueous dispersion of toluidine red containing about 50 percent solids.

EXAMPLES 13 to 16 taerythritol type, acid value l05ll5, specific gravity 1.08, melting point l40-1 C. This resin was cold dissolved in the denatured alcohol, after which the pigment and isopropanol were added and the mixture ball milled for 24 hours. The so produced ink was used in conjunction with each of the printing plates of Examples l to 4, but in each instance positive polarity was applied to the conducting roller in contact with the impression cylinder in order to produce the necessary repelling field.

EXAMPLES 17 to 20 The flexographic printing ink of Examples 13 to 16 was replaced with Magenta pigment 300 grams maleic resin, alcohol soluble 250 grams Denatured alcohol 750 grams In this instance the magenta pigment was of the PT MA type, Color Index No. Pigment Red 81, and the maleic resin was of the pentaerythritol type, melting point l28-l 38 C, acid value 110. The resin was cold dissolved in the denatured alcohol, the pigment added and the mixture ball milled for 24 hours.

This ink was also used in conjunction with positive polarity applied to the conducting roller in contact with the surface of the impression cylinder, with each of the flexible relief printing plates of Examples l2 to 16.

Thus it will be seen that contrary to the prior art methods of electrostatic printing and of electrically or electrostatically assisted printing in which electrostatic or electrical forces were applied to assist ink transfer in printing or image areas, in the method provided by this instant invention electrical or electrostatic forces are employed to suppress or prevent ink transfer in the non-printing areas. Whilst not wishing to be bound by any theory, it is believed that in flexography, the application of electrical or electrostatic forces in accordance with this invention permits transfer of ink from the raised printing areas of the master by virtue of the adhesive forces acting between the ink and the paper or web being greater than the cohesive forces within the ink film carried on such raised portions and being also greater than the repelling force exercised by the applied electrostatic field or potential difference, whilst at the same time said electrical or electrostatic repelling force is greater than the capillary attraction between the paper or printing web and the ink contained in the lower non-printing areas of the relief printing plate.

What I claim is:

1. In a printing process where a web or sheet to be printed moves through the nip of a plate cylinder and an opposed impression cylinder, the steps comprising:

a. selecting an ink having a particular direction of migration under the influence of a directional electrical field;

b. positioning on the plate cylinder a flexible relief printing plate having a resistivity lower than the resistivity to ground of the impression surface of the impression cylinder;

c. positioning said impression cylinder in respect to said plate cylinder to ensure printing contact between the printing surface of said flexible relief plate and a receiving web positioned to move through the nip between said plate cylinder and said impression cylinder;

d. connecting a directional high voltage supply source to apply a directional electrical field between said plate cylinder and said impression cylinder, said directional field being applied in a direction which inhibits migration of said ink from the flexible relief plate towards the web;

. causing said web to move through said nip area at the same speed as the peripheral speed of said relief plate mounted on said plate cylinder whereby ink will transfer to the web by direct contact with the plate; and

f. applying said selected printing ink to the printing plate while applying said directional electric field whereby-ink transfers to the web by contact with the relief printing characters whilst ink is repressed in the surrounding lower areas of the plate.

2. A method according to claim 1 wherein the directional field is applied by the use of a direct current voltage within the range 200 v to 10 k.v. and has a polarity opposite the polarity of a field which induces migration of the ink.

3. A method according to claim 1 wherein said flexible relief plate has a through resistivity not in excess of 7.5 X 10' ohms when measured with electrodes of 1 sq. cm area each placed one on each side of the plate and in contact with the plate.

4. A method according to claim 3 wherein the directional field is applied by the use of a direct current voltage within the range 200 v to 10 k.v. and has a polarity opposite the polarity of a field which induces migration of the ink. 

1. In a printing process where a web or sheet to be printed moves through the nip of a plate cylinder and an opposed impression cylinder, the steps comprising: a. selecting an ink having a particular direction of migration under the influence of a directional electrical field; b. positioning on the plate cylinder a flexible relief printing plate having a resistivity lower than the resistivity to ground of the impression surface of the impression cylinder; c. positioning said impression cylinder in respect to said plate cylinder to ensure printinG contact between the printing surface of said flexible relief plate and a receiving web positioned to move through the nip between said plate cylinder and said impression cylinder; d. connecting a directional high voltage supply source to apply a directional electrical field between said plate cylinder and said impression cylinder, said directional field being applied in a direction which inhibits migration of said ink from the flexible relief plate towards the web; e. causing said web to move through said nip area at the same speed as the peripheral speed of said relief plate mounted on said plate cylinder whereby ink will transfer to the web by direct contact with the plate; and f. applying said selected printing ink to the printing plate while applying said directional electric field whereby ink transfers to the web by contact with the relief printing characters whilst ink is repressed in the surrounding lower areas of the plate.
 2. A method according to claim 1 wherein the directional field is applied by the use of a direct current voltage within the range 200 v to 10 k.v. and has a polarity opposite the polarity of a field which induces migration of the ink.
 3. A method according to claim 1 wherein said flexible relief plate has a through resistivity not in excess of 7.5 X 108 ohms when measured with electrodes of 1 sq. cm area each placed one on each side of the plate and in contact with the plate.
 4. A method according to claim 3 wherein the directional field is applied by the use of a direct current voltage within the range 200 v to 10 k.v. and has a polarity opposite the polarity of a field which induces migration of the ink. 